The present invention relates to the preparation of U.V. and electron beam curable compositions and to the formation of coatings from such compositions.
It is well known that many ethylenically unsaturated monomers may be polymerised upon being suitably activated. This activation may be derived from free radical initiators, by photoinitiation, by thermal initiation or by the use of U.V. or ionising radiation. These polymerisable monomers have been used as coating materials and cured in situ by one of the means referred to above. These coatings have been found to be generally unsatisfactory due to the difficulty of obtaining a complete cure of the coating. It is thought that this difficulty stems largely from the coating layer being too thin to enable a sufficiently high steady state of free radicals to be generated. A further difficulty associated with the in situ curing of the monomers referred to above is that the low viscosity of the monomer makes it difficult to form an even coating on an article and to maintain that coating until curing has taken place.
It has been proposed in U.S. Pat. Nos. 3,551,246 and 3,546,002 to form prepolymers thermally and to form U.V. curable coatings from the prepolymers. These thermally produced prepolymers have been found to be of limited use due to the dark colour generated within the prepolymer during its formation; due to the difficulty of curing the prepolymer; and due to the limited number of monomers and monomer blends which will form curable prepolymers without the precipitation of homopolymer from the mixture.
The present inventors have found that a very large number of prepolymers which are generally clear, highly viscous and water white in colour and which cure readily under U.V. radiation or an electron beam may be prepared by the use of U.V. or ionising radiation initiation.